You can download PDF here. Main idea is to measure wavelength in dielectric without using expensive equipment. Only dielectric resonator is required.
This paper presents a simple method for determining relative dielectric constant of microwave PCB substrates without involving expensive equipment. Microstrip oscillator with elongated feedback arms is etched on a substrate being measured. Dielectric resonator is moved and aligned along feedback arms in different positions. Each position which result in build up of oscillation is marked on PCB surface. Distance between marks is then measured and used to calculate relative dielectric constant.